The present invention relates to radio communication systems using a time division channel multiplex on a carrier frequency.
The time division multiplex may relate to a time-division multiple access (TDMA) scheme, whereby multiple channels are set up between a transceiver, for example a base station of a cellular network, and several transceivers, for example mobile telephones. The time division multiplex may also be used to provide communications in duplex mode between two stations, with time slots allocated alternately in the two directions of communication (TDD—time-division duplex).
In a TDMA or TDD system, a transmitter produces bursts of radio signals in given time slots of a frame structure, allocated to a channel set up with one or more receivers. Such a burst contains a certain number of information symbols to be communicated to the receiver. In many systems, it will also contain training symbols known to the receiver in advance, enabling it to obtain parameters needed to demodulate the received signal in order to estimate the information symbols. These parameters relate to the time and/or frequency synchronisation of the receiver and to the estimation of the propagation channel if the receiver performs coherent demodulation.
In TDMA systems, such as the European GSM radio telephony system, the training symbols form a sequence located in the middle of the burst, so that the demodulation parameters which they enable to be estimated exhibit minimum sensitivity to the variation in time of the propagation channel. In the specific case of GSM, this central training sequence is made up of 26 bits which are known a priori, preceded and followed by information sequences of 58 bits, the 26-bit sequence being used to track the synchronisation of mobiles with the base station and estimate the parameters required by the Viterbi equaliser incorporated in the demodulator.
In radio communication systems which do not use time-division multiplexing, receivers have been proposed which demodulate a segment of the received radio signal twice, once in the direction of increasing times (forward) and once in the direction of decreasing times (backward), in order to enhance the reliability of the estimated information symbols. In each direction, the demodulation involves estimating demodulation parameters at one end of the segment and scanning from this end of the segment to the other end in order to estimate the transmitted symbols. The two sets of estimated symbols thus obtained can then be combined in order to improve the binary error rate on the channel as compared with demodulation in a single direction. A training sequence is inserted at regular intervals in the continuous flow of symbols modulated on the carrier in question and the segment processed by the receiver in the two directions starts and ends with the signals corresponding to two consecutive occurrences of the modulated training sequence used to estimate the demodulation parameters. Examples of such forward/backward demodulators are described in EP-A-0 821 500 and EP-A-0 821 501.
This forward/backward demodulation makes the receiver less sensitive to deep fades in the channel due to destructive interference between the multiple propagation paths. However, the way it is applied is not suitable for time-division multiplexed radio signals.
An object of the present invention is to enable forward/backward modulation methods to be applied in such time-division multiplex systems.